The only way Russia would surrender Crimea would be upon unconditional access to Sebastopol, which the Ukraine denied and gave rise to the conflict currently underway. So, asymptotically close to zero...
Crimea wanted to leave, regardless of access to Sevastopol. Ukraine only denied access, which was guaranteed by treaty, after Russia endorsed the result of the Crimean status referendum.
Correction:
... after the Russian occupation of Crimea, which pre-dated the referendum. I screwed up the history.